1. Field of the Invention
The present invention relates generally to a ceramic pressure sensor, and more particularly to a pressure sensor which has a ceramic diaphragm which is deformed in response to a pressure applied thereto, and at least one resistor which is formed on the diaphragm and whose resistance value varies according to a magnitude of deformation of the diaphragm, and consequently represent the pressure applied to the diaphragm. The present invention also relates to a method of producing a ceramic pressure sensor as indicated above.
2. Discussion of the Prior Art
There is known a pressure sensor or detector of a type wherein resistors connected in a Wheatstone bridge configuration are formed on a diaphragm, which is deformed by a pressure applied thereto. Deformation of the diaphragm results in a certain degree of strain of the resistors, thereby causing a loss of balance of the bridge of the resistors in relation to the pressure applied to the diaphragm. According to this principle, the pressure sensor is capable of measuring a pressure. As a pressure sensor of this type, there is proposed a ceramic pressure sensor which utilizes properties of ceramics, such as high heat resistance, which permits the sensor to measure a pressure at an elevated temperature. Examples of the ceramic sensor are disclosed in SAE Report, 820319 and 860474.
In such a ceramic pressure sensor, a ceramic diaphragm is generally secured on a baseplate, by means of a sealing glass, and the diaphragm is adapted to receive a pressure at one of its opposite surfaces, such that the diaphragm is deformed due to the pressure. As a result, resistors on the deformed diaphragm are given a corresponding strain, which causes a change in the resistance values of the resistors. The pressure applied to the diaphragm is determined by detecting the resistance change of the resistors.
In the conventional ceramic pressure sensors, however, the diaphragm is formed and fired separately from or independently of the baseplate. Namely, the prepared diaphragm is bonded to the separately prepared baseplate by a sealing or bonding material such as a glass. Therefore, there exists a difference in thermal expansion coefficient between the bonding material and the diaphragm, or between the bonding glass and the baseplate. This difference may lead to reducing the strength of the interface between the diaphragm and baseplate, and deteriorating the pressure tightness of the pressure sensor, due to repeated alternate exposures of the sensor to high and low temperatures in pressure measuring cycles. Further, the difference in the thermal expansion coefficient between the bonding material and the diaphragm may unfavorably cause a deformation of the diaphragm and a change in the resistance values of the resistors, even when a pressure is not acting on the diaphragm. Thus, the pressure sensor may have a pressure sensing error, or suffer from lowered accuracy of sensing.